1. Field of the Invention
The present invention relates to a method of producing an electrically conductive film using a melt spray technique.
2. Related Art Statement
Recently, electrically conductive ceramics have attracted attention as functional materials, such as, ion conductive bodies, and electron conductive bodies. This is because ceramics, different from metals, are stable at high temperature atmospheres of around 1,000.degree. C., so that they can be used at various sites for which metals are not usable due to difficulty pertaining to electrical connection at such high temperatures. A typical application example thereof is high temperature type solid electrolyte fuel cells which operate at high temperatures of around 1,000.degree. C. and wholly made of an electrically conductive ceramic. For example, yttria stabilized zirconia used as a solid electrolyte is an oxygen ion conductive body, lanthanum manganate used as an air electrode is an ion/electron mixture conductive body, and lanthanum chromate used as interconnectors or separators for connecting unit cells is an electron conductive body.
As techniques for forming such electrically conductive ceramic films, there can be considered a wet process, such as, doctor blade method, and slip casting method, and a gas phase method, such as, physical vapor deposition method (PVD) and chemical vapor deposition method (CVD). However, a wet process can not cope with complicated configurations, while a gas phase method is slow in film-forming rate, is not suitable for obtaining films of large sizes or large surface areas, and is considerably expensive in production cost.
As a method of removing these drawbacks, a melt spray technique, particularly, a plasma melt spray method, has recently attracted attention for forming electrically conductive ceramic films. This is because films of large surface areas can be formed at a high rate by melt spray methods contrary to the above gas phase method, and still can cope with films of complicated and large shape sizes contrary to the wet process.
For instance, for forming the above high temperature type solid electrolyte fuel cells a method has hitherto been used of using plasma melt spray to form constitutional members of the cells (Sunshine, Vol. 2, No. 1, 19981).
However, electrically conductive ceramic films obtained by plasma melt spray method have generally higher activation energies participating in electric conduction than those of polycrystalline bodies obtained by usual powder sintering methods. This means that, if ceramic films of a same density are prepared by a melt spray method and a powder sintering method, those ceramic films prepared by the melt spray method have lower electrical conductivities than those prepared by the powder sintering method. From a functional aspect of electrical conductivity, this incurs a loss of energy.
In such a case, most of electrically conductive ceramic films show semiconductor-like electrical conductive behaviours, so that a method can be considered of raising the atmosphere temperature to improve the electrical conductivities thereof. However, this method requires an excessive energy for intentionally raising the temperature, hence it is not a practical method. For example, in the case of the above solid electrolyte fuel cells, deterioration of the electrical conductivity of each constitutional element means decrease of output of the fuel cells. Moreover, fuel cells are generally operated at around 1,000.degree. C. at present, and an increase of the operation temperature of the cells causes packaging and gas-sealing of the cells to become more difficult. Therefore, plasma melt spray films having small activation energies and electrical conductivities at high temperatures approximately equal to those prepared by powder sintering method have been requested and a method of producing the same has been earnestly desired.